1. Field of the Invention
This invention relates to an improved exhaust gas recirculation system for circulating a portion of exhaust gases back into an air intake system. Combustion temperature in the recirculation system is kept low in order to decrease the emission of nitrogen oxide and eliminate knocking due to excessively premature combustion.
Conventionally, automobile engines are endowed with an exhaust gas recirculation (EGR) system which recirculates inert exhaust gases into an intake system in order to keep combustion temperatures relatively low. This helps prevent nitrogen oxide emissions from being produced and discharged. Such an automobile engine is also endowed with a positive crankcase ventilation (PCV) system for recirculating what is known as "blowby" gas. Compressed and/or combustion gas, blown through cylinders into an engine crankcase, is introduced into the intake system so as to prevent it from escaping into the atmosphere.
2. Description of Related Art
In recent years, such engines have also been equipped with superchargers. A turbine of such a supercharger is either driven by the energy of exhaust gasses from the engine or mechanically driven by the engine so as to supply compressed air into the intake system for the purpose of increasing an air charging rate and thereby improving the output performance of the engine. Of course, the use of a supercharger increases the air pressure in an air intake pipe of the intake system while supercharging occurs. Various types of exhaust gas recirculation (EGR) systems and positive crankcase ventilation (PCV) systems, have been proposed to efficiently introduce exhaust gases and blowby gases into an intake pipe with high pressure air. Examples of such systems and positive crankcase ventilation (PCV) systems are known from, for instance, Japanese Patent Application No. 58-29,699, entitled "Blowby Gas Recirculation System For Engine With A Supercharging Feature," filed on Feb. 23, 1983 and now opened to public as Japanese Unexamined Patent Publication No. 59-155,520, and Japanese Utility Model Application No. 55-7,350, entitled "Exhaust Gas Recirculation System For Engine," filed on Jan. 23, 1978 and now opened to public as Japanese Unexamined Utility Model Publication No. 56-109,646.
In the exhaust gas recirculation systems described in these publications, however, exhaust gases, when circulated into the air intake systems, leave carbon deposits on the inside walls of intake pipes of the air intake systems. When carbon masses are separated from the pipe walls or shaken loose, possible damage to the engines can occur. If an exhaust gas recirculation system is used with an engine having a supercharger and directs exhaust gases into an intake pipe upstream of the supercharger, the system can still introduce exhaust gas with high efficiency even at high supercharging rates. Nonetheless, such an exhaust gas recirculation system produces carbon deposits which seep into very narrow openings inside the supercharger, thereby decreasing the operational reliability of the supercharger. The decrease in operational reliability is particularly critical with a positive-displacement type of supercharger.